Use a food processor or other kitchen tool to cut potatoes to about 1/8 inch thick. I used the thick slicer attachment on my KitchenAid mixer. Layer potatoes into 2 (9 ounce) ramekins or other baking dishes.
In a small saucepan, bring cream to a simmer over medium heat. Add nutmeg, thyme, and salt and pepper and stir to combine.
Place ramekins over a sheet pan to catch any drippings. (Optional: line with foil or parchment paper for easier clean-up.) Carefully pour half of cream over each dish of potatoes. Sprinkle 3 tablespoons shredded gruyere cheese over the top of each dish.
Bake for 30 minutes, until potatoes are tender. (While potatoes are baking, start making the salmon.) Let rest 10 minutes before serving.
Preheat oven to 400 degrees (if not already done for the potatoes). Spray a baking dish with cooking oil spray, or line with parchment paper or foil (and then spray) for easier clean-up.
Prepare salmon: Season salmon with salt and pepper. Spread flour out on a plate, and put the egg in a shallow bowl or rimmed plate. Spread out pecans on a third plate. Dip the top (skinless side) only of each fillet into flour. Then dip the top side into the egg, then into the pecans. Press gently to help pecans stick to the top of the salmon. Place salmon on prepared baking dish. Drizzle 1 tablespoon maple syrup over each fillet.
Cook salmon: When there are about 5 minutes left on the potatoes, transfer salmon to oven. (It will finish cooking during potatoes’ resting time.) Bake for 10-15 minutes, depending on thickness, until fish flakes easily or reaches internal temperature of 145 degrees.
Make spinach: While the salmon is baking, make spinach. Heat olive oil in a skillet over medium heat. Add spinach and cook until wilted, stirring as spinach cooks down. Season with salt and pepper.
Spoon any maple syrup left in the baking pan over the salmon before serving.
*1 potato may be enough for the gratins, but have 2 on hand in case you need more.